Is the 256MB size of NVIDIA 320M fixed or dynamic?

I'm going to use a Macbook Air with 4GB de RAM, and I need to know if the shared memory of the 320M GPU (256MB according to the Apple website) can grow on demand, or if it's fixed, or if there's some OSX setting where I can allocate it.

I'm going to use a Macbook Air with 4GB de RAM, and I need to know if the shared memory of the 320M GPU (256MB according to the Apple website) can grow on demand, or if it's fixed, or if there's some OSX setting where I can allocate it.

"According to Notebookcheck.net, the 320M does not have dedicated graphics memory, but uses shared memory for the system for its graphics processing, giving it poorer performance than a GPU with dedicated memory."